Carbonaceous compaction using high temperature fluid coke



United States Patent 3,427,240 CARBONACEOUS COMPACTION USING HIGHTEMPERATURE FLUID COKE Thomas Collum Landrum and Robert Harry Waghorne,

Baton Rouge, La., assignors to Esso Research and Engineering Company, acorporation of Delaware No Drawing. Filed May 17, 1966, Ser. No. 550,591U.S. Cl. 204-294 5 Claims Int. Cl. B01k 3/08 Flhis invention relates tocarbonaceous electrodes, espe cially to the preparation of electrodesfrom fluid coke. More particularly, it relates to the preparation ofdustingresistant electrodes from high temperature fluid coke and carbonblack.

Carbon electrodes are used in large quantities by the aluminum industryin the electrolysis of alumina by the Hall process to make metallicaluminum. Such electrodes must meet strict requirements in order to giveeconomical trouble-free operation. They must have high electricalconductivity, good resistance to cracking and must be consumed at auniform rate in order to prevent dusting, i.e., sloughing off ofcarbonaceous particles.

In the past electrodes have been manufactured from coke formed atrelatively low temperatures, generally below about 1200 F. A common formof low temperature coke is that known as delayed coke. In themanufacture of such coke, petroleum or other heavy hydrocarbons arecracked at low temperatures, i.e., from about 750 F. to about 950 F., toform coke and relatively light hydrocarbons. 'The heavy hydrocarbons aredecomposed in a coking chamber, the coke accumulating therein while thelight hydrocarbon products are evolved as a gaseous effluent. Thedelayed coke is removed rfr-om the chamber only with .considerabledifiiculty, i.e., by actual physical dislodgment with crow bars,hydraulic jets and the like. In the process, the coking chambers areusually employed in pairs, one chamber being used for collecting cokewhile coke is being dislodged from the other.

When using coke from the delayed coking processes, it is necessary firstto calcine the coke at 2000 F. to 2400 F. to eliminate volatiles anddensity the carbon. Then it is necessary to grind the coke to varioussizes and mix the various size range fractions together with :a binderto produce green carbon electrodes. These green electrodes arerecalcined to produce the final, electrically conductive electrodes.(The delayed coke is of different sizes, and pieces up to about one inchare used in the mixture. The ground delayed coke particles arenon-uniform and have jag ed edges and irregular surfaces with some largepieces and some extremely fine pieces which help to give a good compactmixture in the manufacture of carbon electrodes, providing a wideparticle size distribution is used.

While delayed coke has admirable characteristics making it highlysuitable as a raw material 'for manufacturing electrodes, thedifliculties associated with the process of manufacture leave much to bedesired. Among other things, the difliculties associated with theprocess m ake delayed coke quite expensive. Moreover, because a highquality feedstock is required, delayed coke is in short supplyworldwide.

It has long been desirable to manufacture electrodes from a moreconveniently available and less expensive form of low temperature cokeand, in this regard, a limited degree of success has been achieved.Thus, e.g., U.S. Patents, 2,881,130, 3,043,753, and 3,197,395 describethe production of low temperature fluidized coke, i.e., coke formed bycontact of hydrocarbons with hot fluidized coke solids at temperaturesranging from about 850 F. to about 1200" F., and the formulation of suchcoke, in

whole or in part, with mixtures for forming electrodes.

3,427,240 Patented Feb. 11, 1969 Low temperature fluid coke has onlylimited utility and has not been generally accepted by carbon electrodemanufacturers because it has been found not to meet stringent criteriaon, e.-g., conductivity and dust-free operability except when oarettullytailored to critical particle size distributions. Such electrodes, forexample, are described in U.S. 3,197,395 where acceptable electrodes areonly produced by using a mixture of three distinct particle sizefractions of low temperature coke.

Because of these process inconvenieuFes and expenses the use inelectrode manufacture of low temperature fluid coke has been quitelimited, and the major source of electrode coke continues to be delayedcoke.

Coke from a recently developed high temperature fluidized coking processcould otter process advantages. This coke, formed only at temperaturesabove about 1 800 E, however, has properties which are in sharp contrastwith those of delayed coke and low temperature fluid coke, supra. Hightemperature fluid coke can be produced, 'for example, by the recentlydeveloped process disclosed in U.S. patent application S.N. 333,897 andnow Patent No. 3,264,210. The differences in properties of coke formedat temperatures above 1-800 compared to delayed coke and low temperaturefluid coke are apparently due to the tact that at the high temperaturethe coke ct'orms entirely by a gas phase reaction whereas in lowtemperature processes the coking reaction occurs in the liquid phase.

Since the high temperature coking process is applicable to the very lowmolecular weight feeds, e.g., methane, as well as high molecular weightfeeds such as residua, and achieves much higher conversions than can beattained by either of the delayed coking or low temperature fluid cokingprocesses, it would appear highly desirable to use the high temperaturecoke product in the manufacture of electrodes. This, of course, washeretofore considered to be impractical due to certain unique propertiesof high temperature fluid coke.

Thus, it has heretofore been considered impractical to producecommercially acceptable alumina reduction electrodes of either theprebaked or Soderberg types from high temperature fluid coke. It isknown, e.g., that at the temperatures at which electrodes are prebakedor used, the binder material in the electrodes forms a type of coke verymuch like low temperature cokes and considerably different from hightemperature fluid coke. The incompatibility of the binder coke and thehigh temperature fluid coke constituents is known to cause certainserious defects in the electrode. Thus, micro cracks form in theelectrode due to the pulling apart of the binder from the fluid cokeparticles, which results from differences in adhesion and thermalexpansion characteristics of the two types of coke. Moreover, the hightemperature coke particles are extremely hard and have a low surfacereactivity, in sharp contrast to the soft binder coke. Electrodesresulting from ingredients having such combination of characteristicsare known to have a tendency to dust severely, i.e., slough offcarbonaceous particles and cause the electrodes to break downprematurely in the alumina bath. This unduly increases electrodeconsumption and can result in short circuiting the bath. The dusting isbelieved to be caused by the selective action of evolved oxygen on thelower density, more reactive coke material derived from the binder inthe finished electrode as compared to the carbon from the fluid coke.Although some dusting occurs with low temperature fluid coke elec'trodes, the problem is much more extreme in high temperature fluid cokebecause the latter is formed at the high temperatures as very hard,small spheres having an extremely hard surface with correspondingly lowreactivity.

In view of the disadvantageous dusting characteristics of hightemperature coke, it has heretofore been thought impossible to preparecommercially acceptable electrodes from this type of material. Such useof high temperature fluid coke was considered particularly unlikelysince even certain forms of low temperature coke, i.e., low temperaturefluid coke, have had only limited applicability in electrodes.

Surprisingly, however, it has now been found that high temperature fluidcoke can be employed pursuant to the present invention to makesatisfactory electrodes possessing minimal tendency toward dusting, andthis can be done without laboriously tailoring the particle sizedistribution as required for low temperature coke. Further, inaccordance herewith it is even feasible to greatly reduce or completelyeliminate the necessity of grinding the high temperature fluid cokeparticles to produce fines. Furthermore, it has been unexpectedly foundthat electrodes of high crushing strength can be made from the hightemperature coke using very low percentages of the relatively expensivecarbonaceous binder.

In accordance with this invention high temperature fluid coke, carbonblack, and a carbonaceous binder are mixed and formed into compactionssuitable for use in alumina reduction electrodes. The compactions can beproduced by admixing 100 parts high temperature fluid coke with fromabout 5 to about parts by weight of carbon black and from about 10 toabout 30 parts of binder followed by compacting the mixture in anyconvenient manner, e.g., by using a conventional briquetting machine atabout 160 F. to 300 F. or a hydraulic press operated at about 2000 to20,000 psi. The compactions can then undergo conventional cures, e.g.,by baking at temperatures ranging from about 1200 F. to about 2600 F.for about 48 hours to 14 days to coke the binder material and produceprebaked electrodes. Alternately, the compactions can, of course, beused in Soderberg electrode formulations without curing, since suchelectrodes are cured in situ in the alumina reduction cell. In eithercase the cured compactions comprise high temperature fluid coke, thecoked residue of carbonaceous binder, and carbon black.

Optimum benefits of the carbon black are attained using about 10 partsof black per 100 parts of coke; however, lesser amounts may be employedwith, of course, correspondingly reduced effectiveness. Electrodeimprovement, however, is nil below about 2 to 3 parts black. Similarly,greater amounts of carbon black can be used to impart still betternon-dusting characteristics to the electrode; however, it is generallyundesirable to use more than about 10 parts of black per 100 parts cokebecause at above this level the overall electrode resistivity is rapidlyincreased above acceptable levels for use in alumina reduction cells.

Any type of carbon black can be used; however, thermal blacks resultingfrom pyrolysis of hydrocarbons at about 1800' F. to 2400 F. aregenerally preferred. Blacks having an average particle size above about1000 A., and preferably in the range of about 2000 A. to 4000 A., willgive satisfactory improvement in the electrode resistance to dusting.Such blacks are also most economical to use since they can be producedas a by-product in high temperature fluid coking processes.

The carbon black can conveniently replace ground coke fines in specificformulations where such fines are called for, thus reducing oreliminating the expense of grinding coke. Moreover, a sometimesdesirable increase in electrode density is obtained when carbon black isadded to the formulations. Thus, in a typical example, when 10 wt.percent carbon black was added in place of a corresponding quantity ofground coke fines, the resulting electrode density was about 1.66 g./cc. compared with about 1.50 g./cc. when carbon black was omitted.

While applicants do not wish to be bound by any particular theory, it isbelieved that the salutary effect of carbon black is in part due tocertain similarities between its surface characteristics and those ofhigh temperature fluid coke. Thus, both are formed by gas phasereactions at the same temperatures, and both tend to have higherelectrical contact resistivities than low temperature fluid or delayedcokes. Similarly, the carbon black and high temperature fluid cokegenerally have higher contact resistivities than the coke which formsfrom the electrode binder material when it is baked. Thus, when carbonblack is not present current tends to pass into the alumina reductioncell through the low resistivity binder coke in preference to the hightemperature fluid coke of the electrode, and the result is that thebinder coke reacts and is used up at a faster rate than the fluid coke.This is believed to increase the tendency for the electrode to dust.

When carbon black is added, however, it apparently tends to change thecharacter of the binder coke giving the latter a contact resistivityapproximately the same as that of the high temperature fluid coke. Thus,the electrical current density tends to be more nearly uniformthroughout the electrode with the result that there is no preferentialdestruction of binder; dusting is reduced, and, inter alia, electrodelife .is correspondingly increased.

A feature of the invention provides for the use of from about 10 toabout 12 parts of carbonaceous binder per parts of high temperaturefluid coke in prebaked electrodes because this gives a much better bakedcompaction with respect to the density, electrical resistivity, andcrushing strength than when greater quantities of binder are used. Thisis surprising because generally about 17 to 20 parts binder per 100parts of coke are required when low temperature delayed coke is used.The savings in binder which is possible using high temperature fluidcoke is particularly significant since the binder is often moreexpensive than the coke.

Any conventional carbonaceous binder can be used, including, e.g., coaltars, petroleum pitch and asphalt binders.

The high temperature fluid coke for use in making the electrodes of thisinvention is prepared in the conventional manner from petroleumfeedstocks at about 1800 F. to 2500" F. Typical high temperature fluidcokes have the following particle size distributions:

Cumulative percent on screen Mesh size (Tyler) It is generally desirableto use the larger particle sizes; however, any of the products producedunder the conventional high temperature coking conditions can be used.

Part of the high temperature fluid coke may be finely ground prior tousing it in the electrode composition. Any conventional grindingtechnique can be employed, and any fraction of the coke used in theelectrode up to about 50% may be ground to fines. Fines as used hereinmeans coke ground sufliciently small that substantially all will passthrough a 200 mesh screen (Tyler) and 40% will pass through a 325 meshscreen.

Also, a part of the fluid coke can be added to the compaction admixtureas preformed agglomerates. Thus, agglomerates of fluid coke can beformed by mixing 100 parts of coke with from about 9 to about 15 partsby weight of a carbonaceous binder, which binder can be the same ordifferent from the carbonaceous binder to be used in forming the finalcompaction. The agglomerates. can then be formed in any conventionalequipment, e.g.,

extruders, briquetters, etc., and then preba-ked in the manner describedabove for the final compactions.

Up to about 75% of the fluid coke in the final compaction can be addedas preformed agglomerates. The agglomerates should be no greater thanabout mm. in size and preferably Will be in the size range of about 1.5to 10 mm., being produced directly within such size range or by crushinglarger agglomerates, e.g., used electrode butts.

Although any conventional mixing technique may be employed, highestquality electrodes are produced if the carbon black is mixed with thecarbonaceous binder material prior to mixing in the high temperaturefluid coke. This technique provides intimate contact between binder andcarbon black and may result in a binder coke more uniformly modified bythe black than would otherwise be possible. Another variation of theinvention is to include part or all of the carbon black in the performedagglomerates rather than adding it separately in the final mixing stepbefore formulation of the electrodes.

In a specific example of this invention 12 parts by weight of coal tarpitch were mixed with 10 parts of thermal black having a particle sizesubstantially in the range from about 1000 to 4000 A., averaging about2500 A. To this mixture were then added 70 parts of unground hightemperature fluid coke substantially in the size range from about 40 toabout 400 microns and averaging about 100 microns. Twenty parts of cokefines were also added to the mixture, substantially all the fines beingground sufficiently small to pass through a 200 mesh (Tyler) screen, and40% of which were sufliciently small to pass through a 325 mesh screen.

The binder, carbon black, fines, and unground coke were then mixed forabout 60 min. at about 325 F., compressed into electrodes in aconventional laboratory electrode press, and prebaked for 48 hours attemperature up to about 2000 F. to coke the binder material.

The electrodes were then tested in a cryolite bath at 1800 F. underconstant electrical current load. After 6 hours electrode consumptionwas 143% of the theoretical carbon consumption for the amount ofelectrical energy use.

In sharp contrast, when electrodes were prepared exactly as in theforegoing example except that the carbon black was replaced byadditional ground coke fines, test results showed that electrodeconsumption in 6 hours was 150% of theoretical, an increase of 7% due toincreased dusting.

It will be apparent that many variations are possible without departingfrom the spirit of this invention.

What is claimed is:

1. A carbonaceous compaction suitable for dustingresistant aluminareduction cell electrode use comprising high temperature fluid coke,carbon black in an amount ranging from about 5 to about 10 parts per I00parts by weight of high temperature fluid coke, and a carbonaceousmaterial selected from the group consisting of carbonaceous binder andthe coked residue thereof.

2. The compaction of claim 1 wherein said carbonaceous material isselected from the group consisting of carbonaceous binder in an amountranging from about 9 to about 15 parts per parts by Weight of hightemperature fluid coke and the coked residue thereof.

3. The compaction of claim 1 wherein said high temperature fluid cokecomprises a mixture of ground coke of sufliciently small particle sizeto pass through a 200 mesh screen and unground coke ranging from about40 to about 400 microns in size.

4. The compaction of claim 1 wherein said high temperature fluid cokecomprises a mixture of agglomerates of coke ranging from about 1.5 toabout 10 mm. in size and unground coke ranging from about 40 to about400 microns in size.

5. The compaction of claim 1 wherein said high temperature fluid cokecomprises a mixture of ground coke of sufliciently small size to passthrough a 200 mesh screen, unground coke ranging from about 40 to 400microns in size, and agglomerates of coke ranging from about 1.5 toabout 10 mm. in size.

References Cited UNITED STATES PATENTS 3,197,395 7/ 1965 Nelson 204-2943,284,334- 11/1966 Metrailer 204-294 3,322,663 5/1967 Loevenstein204-294 JOHN H. MACK, Primary Examiner. D. R. JORDAN, AssistantExaminer.

U.S. Cl. X.R.

1. A CARBONACEOUS COMPACTION SUITABLE FOR DUSTINGRESISTANT ALUMINAREDUCTION CELL ELECTRODE USE COMPRISING HIGH TEMPERATURE FLUID COKE,CARBON BLACK IN AN AMOUNT RANGING FROM ABOUT 5 TO ABOUT 10 PARTS PER 100PARTS BY WEIGHT OF HIGH TEMPERATURE FLUID COKE, AND A CARBONACEOUSMATERIAL SELECTED FROM THE GROUP CONSISTING OF CARBONACEOUS BINDER ANDTHE COKED RESIDUE THEREOF.